Decoupling of metamorphic zircon U-Pb ages and P-T paths in the Dunhuang metamorphic complex, northwestern China

U-Pb dating of metamorphic zircon is commonly used to determine the timing of high-grade metamorphism, but linking zircon U-Pb ages to metamorphic P-T paths can be challenging, especially in metamorphic terranes where multiple metamorphic events result in various degrees of re-equilibration and complex metamorphic zircon growth/recrystallization. Here we carried out a detailed P-T path study, as well as zircon/titanite/apatite U-Pb dating and mineral Sm-Nd isochron dating, for garnet amphibolites from the Dunhuang Block, northwestern China, where late Paleoproterozoic metamorphism was extensively overprinted by a Paleozoic tectonothermal event and the P-T conditions of the earlier event remain elusive. Petrographic observation, phase equilibria modeling and geothermobarometry show that the garnet amphibolites share a clockwise P-T path, from 675 - 689 degrees C/10.0 - 11.6 kbar to 780 - 807 degrees C/9.9 - 11.8 kbar for the prograde to peak metamorphism, followed by significant cooling and decompression to 612 - 689 degrees C/4.1 - 6.0 kbar, indicating an orogenic setting. U-Pb dating of metamorphic zircons yields two groups of upper intercept ages of ca. 1.82 - 1.85 and 1.95 Ga. The ca. 1.82 - 1.85 Ga metamorphic zircons have flat HREE patterns and weak Eu* anomalies, suggesting zircon growth when abundant garnet but little plagioclase was present, consistent with high-pressure metamorphism. In contrast, the ca. 1.95 Ga zircons have steep HREE and negative Eu anomalies, suggesting formation at relatively low -pressures. These zircon results provide evidence for 1.8 - 1.9 Ga metamorphism, possibly due to variable crustal thickening during a late Paleoproterozoic orogenic event. However, garnet porphyroblasts and the matrix minerals yield a Sm-Nd isochron age of 333.5 +/- 4.0 Ma, whereas metamorphic titanites record a U-Pb age of 345.4 +/- 2.7 Ma. Considering the high closure temperatures of the garnet Sm-Nd (650 ??? 720??C) and titanite U-Pb (710 - 780 degrees C) systems, our data suggest that the late Paleoproterozoic metamorphic mineral assemblages were likely completely re-equilibrated by a Paleozoic tectonothermal event and that the P-T path documented above is a result of Paleozoic orogeny. In addition, apatite U-Pb dating yields 236.6 +/- 4.3 and 300 +/- 10 Ma for samples from the southern and central Dunhuang area, respectively, indicating different cooling histories. Combining with previous results, we propose that the Paleozoic metamorphism occurred in a long-lived hot continental arc with slow cooling rates (0.3 - 5 degrees C/Myr), rather than an subduction???accretionary complex. Our study highlights the decoupling of metamorphic zircon U-Pb ages and P-T paths, which has significant implications for P-T-t path studies in polyphase metamorphic terranes.

Automated summary

This study examines garnet amphibolites from the Dunhuang Block in northwestern China, revealing two distinct metamorphic events in the late Paleoproterozoic and Paleozoic based on mineral dating, with evidence of different P-T conditions.